Recently, a so-called RFID (radio frequency identification) system using a contactless IC card, IC tag or the like has been introduced into the fields of art such as an automatic ticket checker used in the railway station, security system for checking people going to enter or exit from a building, electronic money system, etc. As schematically illustrated in FIG. 1, the RFID system includes a contactless IC card 100 and a reader/writer 101 to write and read data to and from the IC card 100. The RFID system adopts the theory of electromagnetic induction. An electromagnetic field radiated from a loop antenna 102 provided at the reader/writer 101 is coupled by the electromagnetic induction to a loop antenna 103 provided at the IC card 100 to provide communications between the IC card 100 and reader/writer 101.
In the above RFID system, the IC card has not to be inserted into the reader/writer to put metallic contacts into contact with each other as in the conventional contactless IC card systems. Therefore, data can be written to, and read from, the IC card easily and quickly. Also, in the RFID system, the electromagnetic field radiated from the reader/writer 101 provides a necessary power to the IC card 100 and thus any power source such as a battery or cell has not to be provided in the IC card. Therefore, the IC card used in the RFID system can be excellent in maintainability, lower in price and higher in reliability.
In the above RFID system, the loop antenna 102 provided at the reader/writer 101 should be able to radiate an electromagnetic field having a certain degree of magnetic strength in order to assure a satisfactory range of communications between the IC card 100 and reader/writer 101.
Generally, the loop antenna 102 for the reader/writer 101 includes a loop coil 200 formed from a plane winding of a conductor as shown in FIG. 2. The loop coil 200 is formed to have winding sections thereof, opposite to each other across the center of the loop coil 200, disposed symmetrically to be equal in interval and width to each other. It should be noted that actual examples of the loop antenna of this type are found in the Japanese Published Unexamined Patent Application No. 1998-144048 in which there is disclosed an antenna provided at a main unit and connected to a reader/writer module, and also in the Japanese Published Unexamined Patent Application No. 2001-331829 in which there are disclosed antennas AG1, AG2 and AG3 of a reader/writer RW.
In the above symmetric loop antenna 102 for the reader/writer 101, the magnetic field is distributed symmetrically as shown in FIG. 3.
FIG. 4 shows the dependence upon the card position of the strength of a current induced by the loop antenna 102 to the IC card 100. As shown, two communication areas S1′ and S2′ are formed in positions, respectively, opposite to each other across the center of the loop coil 200. More particularly, the communication area S1′ assures an ideal magnetic coupling. Namely, magnetic fields developed at four sides of the loop antenna 102 at the reader/writer 101 are inductively coupled to those developed at four sides of the loop antenna 103 at the IC card 100, opposite to the four sides, respectively, of the loop antenna 102. Outside the communication area S1′, there is an area where magnetic fields crossing the loop antenna 103 at the reader/writer 101 cancel each other in a central area where the magnetic fields developed around the loop antenna 102 at the reader/writer 101 are inverted in direction. In this outer area, the induced current will have a lower level than the necessary level for the communications. Outside the above outer area, there is the communication area S2′ where only one of the four sides of the loop antenna 102 at the reader/writer 101 is coupled to one of the four sides of the loop antenna 103 at the IC card 100. Therefore, the communication area S2′ is narrower than the communication area S1′, and the induced current in this communication area S2′ is smaller than that in the communication area S1′.
Note that in FIG. 4, the origin “0” of the horizontal axis indicates the center of the loop antenna 102 at the reader/writer 101 and the positive-going direction indicates a direction from the center (origin “0”) toward outside of the IC card 100. The vertical axis indicates the strength of the current electromagnetically induced in the loop antenna 103 at the IC card 100 under the action of the magnetic field in the loop antenna 102 at the reader/writer 101. Communications are possible in an area where the strength of the induced current has a value larger than a value indicated with a dashed line s′ in FIG. 4.
Note here that when the communication area S1′ is continuously wider as far as possible outwardly of a point, namely, the origin “0”, where the center of the loop antenna 103 at the IC card 100 coincides with that of the loop antenna 102 at the reader/writer 101, the RFID system will be easier to use.
That is to say, in a direction the origin “0” toward outside of the above conventional loop coil 200, the communication area S1′ is followed by a non-communication area once, and then by the communication area S2′. It is desirable from the practical point of view that no non-communication area should exist between the communication areas S1′ and S2′ or only the communication area S1′ should spread.
The above loop antenna 102 for the reader/writer 101 should not be installed as it is to a housing made of an Mg alloy or the like because it will not normally operate due to the influence of an eddy current or the like. In case the loop coil 200 is installed in a metallic housing 300 as shown in FIG. 5, a magnetic sheet 301 is interposed between the metallic housing 300 and loop coil 200 with a sheet 302 of polycarbonate or the like being disposed as a protective material over the loop coil 200. Also in this case, the loop antenna 102 for the reader/writer 101 cannot efficiently radiate any electromagnetic field to the IC card 100, and thus the range of communications between the IC card 100 and reader/writer 101 is narrow.
Further, in case a resin-made housing 400 is used to house the loop coil 200 as shown in FIG. 6, a spacer 402 has to be disposed between an electric circuit board 401 inside the resin-made housing 400 and the loop antenna 102 to prevent any noise from being electromagnetically induced, which will lead to an increased thickness of the housing 400. Moreover, the conventional loop antenna 102 for the reader/writer 101 is nearly equal in dimensions to the loop antenna 103 at the IC card 100. The conventional loop antenna 102 of this type for the reader/writer 101 can hardly be formed smaller and thinner.
Although the conventional RFID system has the above technical problems, the aforementioned reader/writer 101 is installed on a resin-made or metallic housing of some small portable-type electronic devices for example. In this case, the loop antenna 102 for the reader/writer 101 should be formed equal or smaller in outside dimensions to or than, and thinner than, the IC card 100.
Different from the stationary electronic devices, the small portable-type electronic device is limited in size to assure its portability. Therefore, any well worked-out disposition of the loop coil in a space inside the housing will not effectively solve the problems in assuring a space for the loop coil installed inside the housing, that is enough to inhibit the electromagnetic field radiated from the loop antenna 102 from adversely affecting the electronic circuit board and the like disposed near the inner wall of the housing as well as to prevent the metallic housing from affecting the loop antenna 102. Hence, a demand is made for a new method of preventing the metallic housing from affecting on the loop antenna 102 and the electromagnetic field radiated from the loop antenna 102 itself from affecting the electronic circuit board and the like.
Also, different from the stationary electronic devices, the small portable-type electronic device should be operable with a less power consumption. Therefore, the drive current for the loop antenna 102 cannot be smaller, and thus the loop antenna 102 has to be formed to have a new, high efficiency construction which assures a sufficient magnetic field strength even with a small drive current.
Further, because of the limited location of installation and geometric relation of the reader/writer 101 with location of any other functional part such as a keyboard or the like, a desired position of transmission and reception of the reader/writer 101 cannot always be at the center of the loop antenna 102, which will not meet a demand peculiar to the small portable-type electronic device, such as a requirement for a freedom of setting the transmission/reception position to attain an easier operability of the electronic device itself.
Note here that besides the above working-out of the spatial disposition, there have been proposed techniques of preventing the metallic housing from affecting the loop antenna 102, including, as typical ones, an IC tap antenna made of a plate-shaped magnetic material to suppress the influence of a metallic member (as in the Japanese Published Unexamined Patent Application No. 2001-331772) and a card loader antenna made of a magnetic material to suppress the influence of a metallic member by deflecting the magnetic field in the antenna (as in the Japanese Published Unexamined Patent Application No. 2002-123799).
Any of the aforementioned conventional techniques cannot implement any compact and thin loop antenna optimum for a reader/writer in a small portable-type electronic device material- and space-limited in power consumption and location of installation of the loop antenna.